Digital communication receivers must sample an analog waveform and then reliably detect the sampled data. Signals arriving at a receiver are typically corrupted by intersymbol interference (ISI), crosstalk, echo, and other noise. Thus, receivers must both equalize the channel, to compensate for such corruptions, and detect the encoded signals at increasingly higher clock rates. Decision-feedback equalization (DFE) is a widely used technique for removing intersymbol interference and other noise. For a detailed discussion of decision feedback equalizers, see, for example, Digital Communication Principles by R. Gitlin et al (Plenum Press 1992) and Digital Communications by E. A. Lee and D. G. Messerschmitt (Kluwer Academic Press, 1988), each incorporated by reference herein in their entirety.
Generally, decision-feedback equalization utilizes a nonlinear equalizer to equalize the channel using a feedback loop based on previously detected (or decided) data. In one typical DFE implementation, a received analog signal is sampled after DFE correction and compared to one or more thresholds to generate the detected data. The DFE correction, v(t), is subtracted in a feedback fashion to produce a DFE-corrected signal w(t). A clock, generated from the received signal by a Clock and Data Recovery (CDR) circuit, is generally used to sample the DFE-corrected signal and for the DFE operation. An example of such a receiver is disclosed in “Method and Apparatus for Generating One or More Clock Signals for a Decision-Feedback Equalizer Using DFE Detected Data”, by Aziz et al, U.S. Pat. No. 7,616,686, incorporated by reference herein in its entirety, utilizes a DFE-based phase detection architecture for clock and data recovery of a DFE-corrected signal.
A DFE-based receiver includes an analog front end (AFE), typically used to control the input signal level and equalize for linear, frequency-based distortions in the input signal to the receiver. However, the analog circuitry in the AFE has inherent limitations, one of which is the maximum amplitude the circuitry can handle before significant non-linear distortion occurs. For example, should one or more amplifiers in the AFE begin to saturate, i.e., limit, signals into or out of the amplifiers, nonlinear distortion of the input signal results. This nonlinear behavior is typically measured by specifying the input signal to the AFE that results in a 1 dB compression in the output signal of the AFE compared to a non-compressed AFE output signal. Presence of the nonlinear distortion in the input signal might cause suboptimal adaptation by the DFE to the input signal, resulting in possible poor performance by the receiver, e.g., a high bit error rate. This is particularly problematic in backplane bus communication system where compatibility with a defined standard and high-speed operation are required. For example, a standard referred to as “low-voltage differential signaling” (LVDS) is commonly used for backplane communications. LVDS sets a 350 mV peak-to-peak signal requirement with a common mode voltage of 1.2 V for data signals being transmitted, resulting in a peak voltage of approximately 1.375 volts. Generally, as the data rates increase and transistor sizes shrink to handle the higher data rates, the 1 dB compression point of an amplifier is concomitantly reduced due to supply voltage limitations inherent with smaller transistors. As data rates exceed 2 gigabits/second (Gbps), the semiconductor technology used to implement the receiver handling such high speeds has a maximum supply voltage limit, e.g. 1.5 volts, that begins to approach the amplitude peaks of the signals being received, resulting in significant nonlinear distortion. A typical solution is to attenuate the input signals to well below the AFE's 1 dB compression point to keep the input signals in the AFE's linear range. This will allow the AFE circuitry in the receiver to handle these signals without distortion but reduces the noise immunity of the receiver, degrading its bit error rate (BER).
Thus, it is desirable to provide a method to allow a receiver operate properly with input signals that might cause nonlinear distortion within the receiver.